Electromechanical transducer assemblies



Aug. 5, 1969 G. PEARCE ELECTROMEQHANICAL TRANSDUCER ASSEMBLIES 2 Sheets-Sheet 1 Filed Aug. 28, 1967 R 0 f N E m I Geome 1m Aug. 5, 1969 G. PEARCE ELECTROMECHANICAL TRANSDUCER ASSEMBLIES 2 Sheets-Sheet 2 Filed Aug. 28, 1967 afl Mair/2 M United States Patent 3,460,062 ELECTROMECHANICAL TRANSDUCER ASSEMBLIES George Pearce, London, England, assignor to Smiths Industries Limited, London, England, a British company Filed Aug. 28, 1967, Ser. No. 663,661 Claims priority, application Great Britain, Sept. 2, 1966, 39,445/ 66 Int. Cl. H0411 13/00 U.S. Cl. 340-11 9 Claims ABSTRACT OF THE DISCLOSURE A hollow, cylindrical, electromechanical transducer is situated coaxially within a frusto-conical acoustic reflector, the outer cylindrical surface of the transducer propagating acoustic energy for reflection in a beam from the reflector. Part of the volume within the transducer is filled with pressure-release material, and the remainder of the volume within the transducer defines a cavity that is resonant at the frequency of acoustic energy propagated by the transducer. Acoustic energy propagated in the cavity supplements the energy reflected from the reflector in the formation of the beam.

This invention relates to electromechanical transducer assemblies.

The invention is especially concerned with electro-mechanical transducer assemblies for use in sonar systems.

In the application of sonar systems to deep-water echosounding, or to the examination of strata below water, it is desirable to use a beam of acoustic energy having as low a frequency as practicable, since the lower the frequency, the greater the penetration by the beam. There are, however, practical considerations which place a limit on the minimum frequency that can conveniently be used; one such consideration is the size of transducer required in order to obtain the desired beam-width, such size being dependent upon the relevant wavelength.

One known way of achieving low-frequency operation with acceptable size is to use a cylindrical transducer mounted inside a frusto-conical acoustic reflector and operated in a circumferential mode of oscillation. The use of such a transducer assembly, however, has the disadvantage that acoustic energy is transmitted in a main beam that is accompanied by undesirable side lobes,

It is an object of the present invention to provide a transducer assembly which may be used to overcome this disadvantage.

According to the present invention, there is provided an electromechanical transducer assembly for transmitting a beam of acoustic energy into a surrounding medium, comprising an acoustic reflector, and an acoustic transducer for propagating acoustic energy for reflection from the reflector, said transducer having associated therewith a cavity resonant at the frequency of acoustic energy propagated by the transducer for propagating further acoustic energy originating from the transducer, to supplement in the formation of said beam the acoustic energy reflected from the reflector.

The transducer may be of hollow cylindrical form with the cavity therein, and the reflector may have a frustoconical reflecting surface coaxial with the transducer. The axial length of the cavity may be substantially half the wavelength of the acoustic energy transmitted by the assembly into said surrounding medium. The axial length of the transducer may, however, be substantially greater than one half-wavelength, and in this case the volume within the transducer, apart from said cavity, may be filled with pressure-release material.

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The transducer may comprise a plurality of annular magneto-strictive members that are stacked one upon another with interposed rings of pressure-release material, and an electrical winding may embrace the transducer toroidally.

The reflector may be provided by acoustically-reflective material carried by a substantially-closed frusto-conical shell of the assembly, the larger circular end face of the shell being of acoustically-transmissive material.

Although the invention is described above in the context of transmission of a beam of acoustic energy, the invention is equally applicable to the reception of such a beam. In addition it will be appreciated that reference to acoustic energy is not intended to imply limitation to energy Within the audible range of frequencies.

An electromechanical transducer assembly according to the present invention will now be described, by way of example, with reference to the accompanying drawing, in which:

FIGURE 1 is a side-elevation, partly in section, of the transducer assembly;

FIGURE 2 is a directivity characteristic for radiation and reception of acoustic energy in a plane containing the axis of symmetry of the assembly, as applicable to only a part of the transducer assembly of FIGURE 1;

FIGURE 3 is the corresponding directivity characteristic applicable to the whole transducer assembly of FIG- URE 1.

The transducer assembly shown in FIGURE 1 is part of a sonar system as used for hydrographic surveying. In such application the assembly is mounted on the hull of a survey ship below the water surface, for use in transmitting pulses of acoustic energy towards the bottom, and also for receiving the resultant echoes from the bottom and its underlying strata. The frequency of the acoustic energy transmitted is in this specific example is 9.6 kilocycles per second (but in the general context of the present invention neel not necessarily be within the audible-sound range).

Referring to FIGURE 1, a hollow, generally-cylindrical electromechanical transducer 1 includes six annular magneto-strictive members 2 stacked one upon another with their axes colinear, and separated from one another by interposed resilient rings 3 of synthetic-rubber. The annular members 2 and the rings 3 in the stack are secured together by nylon cords 4, and are embraced conjointly by an insulated electrical winding 5 of the transducer 1. The winding 5, which is insulated by means of synthetic-rubber, is wound toroidally on the stack of annular members 2 with adjacent turns spaced from one another.

The transducer 1 is housed in the assembly within a glass-fibre shell comprising a cup-shaped reflector 6 and a circular bottom-plate 7. A frusto-conical wall 8 of the reflector 6, which wall is coaxial with the transducer 1 and extends downwardly from a circular top-part 9, has a core 10 of expanded ebonite so as to reflect acoustic energy emitted by the transducer 1. The bottom-plate 7 closes the open mouth of the reflector 6, being clamped to the reflector 6 by a series of bolts 11. Holes 12, 13 and 14 are provided respectively in the wall 8, the bottomplate 7 and the top-part 9 so that the shell is flooded when submerged.

The transducer 1 is clamped within the shell between the bottom-plate 7 and the top-part 9 by means of a stud 15 and cooperating nut 16. The upper end of the transducer 1 is spaced from the top-part 9 by a disc 17 of expanded ebonite and an interposed resilient ring 18 of expanded rubber, while the lower end of the transducer 1 is spaced from the bottom-plate 7 by means of a glassfibre framework 19 and an interposed resilient ring 20 of expanded rubber. The lower end of the transducer 1 bears via the ring 20 on a shoulder of the framework 19,

the shoulder being provided by a glass-fibre annulus 21. Within the framework 19, the annulus 21 is spaced by struts 22 from an annulus 23 that bears upon the bottomplate 7. Part of the framework 19 also extends into the hollow transducer '1, this part consisting of a third annulus 24 spaced by struts 25 from the annulus 21. The rest of the length within the transducer 1, from the annulus 24 to the disc 17, is occupied completely by four discs 26 of expanded ebonite.

The whole assembly is secured to the hull of the ship by means of bolts 27 which extend from the top part 9 of the reflector 6.

The construction of the transducer 1 will now be described in greater detail. Each annular member 2 has a laminated magnetostrictive core constructed from a single length of metal tape (the metal being an iron-cobaltvanadium alloy), wound spirally to form an annular scroll. The scroll is impregnated, while in a vacuum, with a varnish having an epoxy-resin base; the varnish, on setting hard, holds the scroll rigidly circular. The impregnated scroll is heated to about 160 degrees centigrade, and is then dipped briefly into a fluidized bed of epoxy-resin powder so that it thereby acquires a corrosion-resistant coating. The annular members 2, after being stacked together as described above, are wound with the winding 5. A high-current electrical pulse is then passed through the winding so as to impart a permanent magnetic polarization to each member 2.

The dimensions of the scroll in each magnetostrictive member 2 govern its resonant frequency of oscillation under pulsed electromagnetic stimulus. The scrolls are all of substantially the same dimensions as one another, so the transducer 1 is as a whole is resonant to radiate acoustic energy at substantially one frequency, this being substantially the same as the resonant frequency of oscillation of each of the annular members 2.

The lengths of the struts 25 of the spacing-framework 19 are such that the distance between the end-face of the lowest disc 26 and lower end of the transducer 1 is approximately equal to one half of the wavelength in water of the acoustic energy radiated by the transducer 1. The expanded ebonite of the discs 26 provides a pressure-release wall for acoustic energy, the cavity 28 bounded by that part of the framework 19 within the transducer 1 being resonant to the radiated acoustic energy. The length of the cavity 28, between the opposite ends that are respectively open and closed to the acoustic energy, is not exactly equal to one half-wavelength, but is modified by an end-correction dependent upon the internal diameter of the transducer 1 to give the required degree of resonance.

When an electric current pulse is passed through the winding 5 of the transducer 1, magnetrostriction of the scrolls within the annular members 2 causes alternate circumferential contraction and expansion of each member 2 at the resonant frequency; acoustic energy at this frequency is accordingly radiated into the water from both the internal and external surfaces of the transducer 1. The acoustic energy radiated by the external surface of the transducer 1 is reflected by the frusto-conical core of the reflector wall 8 and is thereby directed downwards in a beam parallel to the axis of symmetry of the assembly. A typical directivity characteristic of acoustic energy radiated solely from the external surface, and reflected in this way is shown in FIGURE 2 of the drawing. FIGURE 2 indicates the variation of relative amplitude of this acoustic energy throughout the angular range of :180 degrees from the axis of symmetry, in any place containing this axis. From FIGURE 2 it can be seen that owing to the side-lobes on either side of the main beam, the acoustic energy in a plane normal to the axis is distributed between a central circular zone and concentric annular zones, and that between adjacent zones there are regions into which only an insignificant amount of energy falls.

In previous transducer assemblies, acoustic energy radiated solely by the external surface of a hollow cylindrical transducer has been reflected towards a target area by means of a frusto-conical reflecting surface. In such assemblies it has been the practice to ensure that no energy is radiated from within the transducer itself, and to this end the inner wall of the transducer has been lined with a pressure-release material; the resultant directivity characteristic obtained in these circumstances is substantially that shOWn by FIGURE 2.

In the assembly shown in FIGURE 1, acoustic energy radiated by the internal surface of the transducer 1 is propagated in the resonant cavity 28, and is radiated from the open end thereof in a direction parallel to the axis of the assembly. This energy is additional to that radiated by the external surface of the transducer 1, and is such as to modify the directivity characteristic to the form shown in FIGURE 3. From FIGURE 3 it can be seen that with this modification less energy falls outside the main-beam, that there are fewer effective side-lobes, and that the main-beam is Wider.

With the characteristic of FIGURE 3, substantially the whole of the radiated energy is concentrated into a single circular zone of the bottom under survey, as opposed to a smaller central zone together with concentric annular zones produced by side-lobes. Hence with the transducer assembly shown in FIGURE 1 and including the central resonant cavity 28, the likelihood of errors caused by echoes arising within the side-lobes is reduced. Although the side-lobes are reduced at the expense of increase in width of the main-beam, the disadvantage of this is not significant compared with the advantage of concentrating the energy into the single zone vertically below the survey ship and thereby obviating ambiguity in interpretation of recorded echoes.

A further improvement provided by this transducer assembly is that the mechanical Q factor is lower than in some previous assemblies, making it possible to transmit and receive shorter pulses of acoustic energy than hitherto, thereby improving the acuity of the received information.

The resonant mode of the cavity 28 within the transducer 1 is complex, the diameter of the cavity 28 being significant in relation to the wavelength of the acoustic energy. The cavity 28 is, therefore, not comparable with a simple open-ended resonator of insignificant diameter. In the present case, the length of the cavity is some 3 inches, and its diameter some 6 inches. The choice of these dimensions is conditioned by the elasticity of the cavity walls, and by the effect this elasticity has on the operational parameters within the cavity.

I claim:

1. An electromechanical transducer assembly for transmitting a beam of acoustic energy into a surrounding medium, comprising an acoustic transducer that is re sonant at a predetermined frequency for propagating acoustic energy of said frequency, and an acoustic reflector mounted to reflect as a beam said energy propagated by said transducer, said transducer having therein means defining a cylindrical cavity having opposite ends that are respectively open and closed to said acoustic energy, the closed end of said cavity being substantially planar, said cavity having a substantially constant diameter between its open and closed ends and having a length between its open and closed ends such that the cavity itself is resonant at said frequency to propagate from said open end further acoustic energy supplementing in the formation of said beam said acoustic energy reflected from the reflector.

2. A transducer assembly according to claim 1 wherein said transducer is of hollow cylindrical form, and said reflector has a frusto-conical reflecting surface coaxial with said transducer.

3. A transducer assembly according to claim 2 wherein said transducer has an axial length which is at least half the wavelength of said acoustic energy, and said cavity has an axial length of substantially one half-wavelength.

4. A transducer assembly according to claim '1 wherein the transducer comprises a plurality of annular magnetostrictive members stacked one upon another, and rings of pressure-release material interposed between said annular members.

5. A transducer assembly according to claim 1 wherein an electrical winding embraces said transducer toroidally.

6. An electrimechanical transducer assembly for transmitting a beam of acoustic energy into a surrounding medium, comprising a cylindrical acoustic transducer for propagating acoustic energy, and an acoustic reflector mounted to reflect as a beam said energy propagated by said transducer, said transducer comprising a plurality of annular magnetrostrictive members stacked one upon another, rings of pressure-release material interposed between said annular members in the stack, an electrical winding embracing the stack of annular members toroidally, and pressure-release material filling the interior of said stack apart from a cavity at one end, said cavity being resonant at the frequency of said acoustic energy for propagating acoustic energy to supplement the energy reflected from said reflector in the formation of said beam.

7. A transducer assembly according to claim 6 wherein said reflector has a frusto-conical reflecting surface coaxial with said transducer.

8. A transducer assembly according to claim 6 wherein said cavity has an axial length of substantially one half the wavelength of said acoustic energy.

9. An electromechanical transducer assembly for transmitting a beam of acoustic energy into a surrounding medium, comprising an acoustic transducer of hollow cylindrical form for propagating acoustic energy, an acoustic reflector mounted to reflect as a beam said energy propagated by said transducer, said reflector having a frusto-conical reflecting surface coaxial with said transducer, said transducer having therein a cavity resonant at the frequency of said acoustic energy for propagating further acoustic energy to supplement said acoustic energy reflected from the reflector in the formation of said beam, said transducer having an axial length substantially greater than one half the wavelength of said acoustic energy, said cavity having an axial length of substantially one halfwavelength, and pressure-release material filling, apart from said cavity, the volume within said transducer.

References Cited UNITED STATES PATENTS 2,005,741 6/1935 Hayes 3408 2,753,543 7/1956 Rymes 340--8 2,922,140 1/1960 Levine et al. 340-8 3,243,768 3/1966 Roshon et al. 340--8 3,325,779 6/1967 Supernaw et al. 34010 X RODNEY D. BENNETT, JR., Primary Examiner B. L. RIBANDO, Assistant Examiner 

